1. Field of the Invention
The present invention relates to methods and apparatus for determining the degree of extension of a fluid cylinder. It particularly relates to methods and apparatus for obtaining an electrical signal indicative of the degree of extension of a fluid cylinder and transmitting such signal to a remote location where the signal may be used for various purposes, including the provision of a continuous readout of the cylinder extension.
2. Description of the Prior Art
Fluid cylinders, both hydraulic and pneumatic, are often used in applications where it is desirable to monitor the degree of extension of the cylinder. When the cylinder is located in an inaccessible or hostile environment, it becomes difficult to visually monitor the cylinder extension. For example, in the underwater construction of tunnels, bridges, piers, and the like, the problem of monitoring cylinder extension becomes particularly acute. Heretofore, the common practice has been to send a diver to the location of the cylinder to visually determine the cylinder displacement. It would be desirable in underwater applications, as well as numerous other applications, to have a system for transmitting information indicative of the cylinder extension to a remote location.
A number of systems have been proposed for this general purpose, some using the principle of an elongate transducer coupled to the piston movement, for example, a linear potentiometer in a control circuit that provides a voltage output indicative of the position of the piston. One such system includes the use of a potentiometer in association with the fluid cylinder where the potentiometer resistor is attached to the outside of the cylinder, with the potentiometer sliding contact being connected to the piston rod. This device has the disadvantage of the elements external of the cylinder being subject to mechanical breakage. Furthermore, the external elements are exposed to corrosives and other effects of the environment.
U.S. Pat. No. 3,355,690 is directed to a fluid cylinder system utilizing a sliding potentiometer wherein the potentiometer resistor, in the form of a coil, is disposed in an axial bore formed in the piston and piston rod. A disadvantage of this system is that the electrical components are exposed to the internal working environment of the cylinder where they must withstand the pressure of the hydraulic fluid and the stresses created by movement of the piston.
Other systems utilize reed switches located at spaced locations along the length of the cylinder, the reed switches sensing when the cylinder passes a particular finite location where the switch is located. These systems have the disadvantage of not providing a continuous reading of the piston position.
Yet another system manufactured by Temposonics Incorporated includes an elongate transducer beam which is located in the cylinder, and sealed to withstand the pressure of the hydraulic fluid, and a permanent magnet that is mounted on the moving piston. This system measures the time interval for an ultrasonic pulse to travel along the beam to the position of the magnet. A disadvantage of this system is the need for isolating the beam and its related electronic components from the pressurized hydraulic fluid. Another disadvantage is the high cost associated with the provision of the electronic, ultrasonic and magnetic components of the system.
Despite a long-standing need for a simple, reliable, commercially feasible system for determining the degree of extension of fluid cylinders that are located in inaccessible or hostile environments, no system is commercially available that has met the functional and commercial restraints.